Semiconductor nanocrystals (also known as quantum dot particles) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of semiconductor nanocrystals shift to the blue (higher energies) as the size of the nanocrystals gets smaller.
Semiconductor nanocrystals are nanoparticles composed of an inorganic, crystalline semiconductive material and have unique photophysical, photochemical and nonlinear optical properties arising from quantum size effects, and have therefore attracted a great deal of attention for their potential applicability in a variety of contexts, e.g., as detectable labels in biological applications, and as useful materials in the areas of photocatalysis, charge transfer devices, and analytical chemistry. As a result of the increasing interest in semiconductor nanocrystals, there is now a fairly substantial body of literature pertaining to methods for manufacturing such nanocrystals. Broadly, these routes may be classified as involving preparation in glasses (see Ekimov et al. (1981) JETP Letters 34:345), aqueous preparation (including preparation that involve use of inverse micelles, zeolites, Langmuir-Blodgett films, and chelating polymers; see Fendler et al. (1984) J. Chem. Society, Chemical Communications 90:90, and Henglein et al. (1984) Ber. Bunsenges. Phys. Chem. 88:969), and high temperature pyrolysis of organometallic semiconductor precursor materials (Murray et al. (1993) J. Am. Chem. Soc. 115:8706; Katari et al. (1994) J. Phys. Chem. 98:4109). The two former methods yield particles that have unacceptably low quantum yields for most applications, a high degree of polydispersity, poor colloidal stability, a high degree of internal defects, and poorly passivated surface trap sites. In addition, nanocrystals made by the first route are physically confined to a glass matrix and cannot be further processed after synthesis.
To date, only the high temperature pyrolysis of organometallic reagents has yielded semiconductor nanocrystals that are internally defect free, possess high band edge luminescence and no trapped emission, and exhibit near monodispersity. Additionally, this route gives the synthetic chemist a substantial degree of control over the size of the particles prepared. See Murray et al. (1993), supra. One disadvantage of this method, however, is that the particles are sequestered in reverse micelles of coordinated, hydrophobic surfactant molecules. As such, they are only dispersible in organic solvents such as chloroform, dichloromethane, hexane, toluene, and pyridine. This is problematic insofar as many applications that rely on the fluorescence emission of the semiconductor nanocrystals require that the nanocrystals be water soluble or at least water dispersible.
Although some methods for rendering semiconductor nanocrystals water dispersible have been reported, they are still problematic insofar as the treated semiconductor nanocrystals suffer from significant disadvantages that limit their wide applicability. For example, Spanhel et al. (1987) J. Am. Chem. Soc. 109:5649, discloses a Cd(OH)2-capped CdS sol; however, the photoluminescent properties of the sol were pH dependent. The sol could be prepared only in a very narrow pH range (pH 8-10) and exhibited a narrow fluorescence band only at a pH of greater than 10. Such pH dependency greatly limits the usefulness of the material; in particular, it is not appropriate for use in biological systems.
Other groups have replaced the organic passivating layer of the semiconductor nanocrystal with water-soluble moieties; however, the resultant derivatized semiconductor nanocrystals are not highly luminescent. Short chain thiols such as 2-mercaptoethanol and 1-thioglycerol have been used as stabilizers in the preparation of water-soluble CdTe nanocrystals. See, Rogach et al. (1996) Ber. Bunsenges. Phys. Chem. 100:1772 and Rajh et al. (1993) J. Phys. Chem. 97:11999. Other more exotic capping compounds have been reported with similar results. See Coffer et al. (1992) Nanotechnology 3:69, which describes the use of deoxyribonucleic acid (DNA) as a capping compound. In all of these systems, the coated semiconductor nanocrystals were not stable and photoluminescent properties degraded with time.
Thus, to use these high quantum yield materials in applications that require an aqueous medium, one must find a way of changing the polarity of the organic coating, thereby facilitating the transfer of these particles to water. A great deal of work has been conducted on surface exchange reactions that seek to replace the oleophilic hydrocarbon coating on the nanocrystal surface with a range of bifunctional polar molecules wherein one functional group of the capping molecule bears some affinity for the surface of the nanocrystal, while the other functional group, by virtue of its ionizability or high degree of hydration, renders the nanocrystal water soluble. For example, International Patent Publication No. WO 00/17655 to Bawendi et al. describes a method for rendering semiconductor nanocrystals water dispersible wherein monomeric surfactants are used as dispersing agents, with the hydrophobic region of the surfactants promoting association with the nanocrystals, while the hydrophilic region has affinity for an aqueous medium and stabilizes an aqueous suspension of the nanocrystals. International Patent Publication No. WO 00/17656 to Bawendi et al. describes a similar method wherein monomeric compounds of formula HS—(CH2)n—X, wherein n is preferably ≧10 and X is carboxylate or sulfonate, are used in place of the monomeric surfactants.
Kuno et al. (1997) J. Chem. Phys. 106:9869-9882, Mikulec, “Semiconductor Nanocrystal Colloids: Manganese Doped Cadmium Selenide, (Core) Shell Composites for Biological Labeling, and Highly Fluorescent Cadmium Telluride,” doctoral dissertation, Massachusetts Institute of Technology (September 1999), and International Patent Publication No. WO 00/17656 to Bawendi et al., cited supra, give detailed descriptions of surface exchange reactions designed to improve the water dispersibility of hydrophobic nanocrystals. In general, these references indicate that: exchange of the original hydrophobic surfactant layer on the nanocrystal surface is never quite complete, with retention of only about 10% to about 15% of the surfactant (even after multiple exchange reactions); although never quantitatively displaced, exchange of the original phosphine/phosphine oxide surfactant layer with more polar ligands results in a substantial decrease in quantum yield that is never entirely regained; once dispersed in water, the particles have limited colloidal stability; and attempts to carry out further chemistry with these particles, such as linking them to biomolecules through their pendant carboxyl functionalities, is highly irreproducible and dependent on the size of the nanocrystal.
Thus, there remains a need in the art for a reliable, reproducible method for rendering hydrophobic semiconductor nanocrystals dispersible in aqueous media while preserving the quantum efficiencies of the original particles, maintaining colloidal stability, and avoiding or minimizing any change in particle size distribution. Ideally, such a method would be useful not only with semiconductor nanoparticles, but also with other types of nanoparticles having hydrophobic surfaces, e.g., semiconductive nanoparticles that are not necessarily crystalline and metallic nanoparticles that may or may not be surface-modified.